Field of Invention
This invention relates to a system operating in conjunction with a computer whose central processing unit manipulates data fed thereto by a variable data source to yield at its output port parallel binary-coded data, the system converting this data both to an output frequency in a range suitable for indicating the variable input data, and to an output current proportional thereto.
More particularly, the invention deals with a system of the above-noted type in which the output frequency and the output current are electrically isolated from the computer and its associated data source to avoid any interaction therewith.
In industrial process control operations, it is often necessary to transmit data obtained at various field stations to a remote indicating or control terminal where the information is processed by the central processing unit of a digital computer. The data conveyed from the variable data sources at the field stations to the receiving terminal may be changes in pressure, temperature, flow rate or any other process variable. Such variable data is usually derived by means of a low level instrument which senses the variable and generates a corresponding electrical analog signal.
In a multiplexing system associated with a digital computer the analog data to be processed must first be converted into corresponding digital signals. A multiplexer makes it possible to share a common information path among several groups of input or output digital devices. By such means, data can be transmitted between a digital processing unit and any one of several digital input or output devices.
In many industrial process control applications, an analog output as well as a frequency output to provide a digital readout is required in order to carry out various control functions. Thus in a typical process control loop where the final control element is a valve governed by an analog signal and the related recorder is a moving chart pen recorder which also requires an analog operating signal, the digital output from the computer which is a function of the sensed input data from the control loop must be converted into analog form. In the United States, the most commonly accepted analog output for process control and recording purposes is a current lying in a 4 to 20 mA (dc) range. Also in use, however, is a 10 to 50 Ma range as well as a zero-based current output, such as 0 to 16 mA and 0 to 20 mA.
The frequency output is not only necessary to afford a digital readout of flow rate or whatever other variable is being monitored, but also for purposes of totalization, such totalization being necessary for billing or other purposes to determine the total flow that has passed through the meter.
In a computerized system of the above type, it is desirable to electrically isolate both the frequency output and the current output from the computer and its associated variable data source to avoid any interaction therebetween. Industrial applications which make use of a flowmeter as the variable data source often require an isolated frequency and current output for accurate flow rate indication and totalization. Since accurate outputs require 12 to 16 bits of precision, isolation of the individual bits of parallel binary-coded data yielded by the microprocessor is an unrealistic and impractical solution to this problem.